The invention relates to a disk apparatus for positioning a head on the basis of a servo pattern recorded on a servo surface and, more particularly, to a disk apparatus in which an offset amount is measured on the basis of a servo pattern for offset recorded on a specific cylinder on a data surface and the position of the head is corrected in such a direction as to eliminate the offset in the reading or writing operation, thereby on-tracking the head.
Hitherto, according to the positioning control of a head of a magnetic disk unit, a servo signal recorded on a servo surface of a disk medium is read and the head is positioned to a track center. Further, even when the servo surface and the data surface are located at the same cylinder position, there is an offset due to a mechanical error, decentering, or the like. Therefore, a servo pattern for detecting the offset is previously recorded at a specific cylinder position on the data surface. When a power source is turned on, the offset is measured for every data surface and stored into an RAM. In the actual reading or writing operation, the offset measurement value is read out from the RAM and the position of the head is corrected, thereby accurately on-tracking the head.
FIG. 1A shows servo patterns for offset detection which are recorded on a specific cylinder of a data surface at regular intervals in a conventional apparatus. It is now assumed that an AGC pattern 210, an inner offset pattern 220, and an outer offset pattern 230 are sequentially recorded along a track center 250 of the specific cylinder on the data surface in a relative moving direction (downward) of a data head 240. Those servo patterns are magnetic recording patterns which are recorded by frequency signals having frequencies higher than a recording frequency. The AGC pattern 210 is the recording pattern which is symmetrical for the track center 250. The inner offset pattern 220 is the pattern recorded to have almost the same width as that of the data head 240 from the track center 250 toward the inner side. The outer offset pattern 230 is the pattern recorded so as to have almost the same width as the data head 240 from the track center 250 toward the outer side.
A measuring process of an off-track is executed as follows. First, a reference value of an amplitude level of an AGC amplifier which is used to amplify a read signal of the data head 240 is determined from an AGC signal V.sub.AGC obtained by reading the AGC pattern 210. As shown in FIG. 1B, the AGC signal V.sub.AGC is a signal which is constant on both sides of a track center P-II and is attenuated to 0 at a position P-I on the outer side or a position P-III on the inner side.
Subsequently, the inner offset pattern 220 recorded on the inner side from the track center 250 and the outer offset pattern 230 recorded on the outer side are respectively read, thereby obtaining offset read signals V10 and V11. An offset amount is obtained on the basis of a difference signal (V10-V11) between the two offset read signals V10 and V11. That is, the offset read signal V10 changes as shown in FIG. 1C and the read signal V11 changes as shown in FIG. 1D in accordance with the offset position of the data head 240. Therefore, the difference signal (V10-V11) between both of those signals is a signal which is proportional to the offset amount and which is set to 0 at the track center 250 and linearly changes in the plus direction on the inner side and in the minus direction on the outer side as shown in FIG. 1E. The offset can be obtained from the difference signal.
The AGC pattern 210 and offset patterns 220 and 230 in FIG. 1A are recorded at regular intervals on a track of a specific cylinder of the data surface. Therefore, as a measurement result of one circumference of the track, an AGC amplitude level and an offset amount are stored into an RAM table or the like. In the head positioning control in the reading or writing mode, the AGC amplitude level and the offset amount are read out from the RAM table synchronously with the disk rotation, thereby setting the level of the AGC amplifier and performing the offset correction of a head positioning signal. FIGS. 2B to 2H show changes in amplitude levels of the read signals V.sub.AGC, V10, and V11 in the case where the head is offset to positions P1 to P7 in FIG. 2A.
In the recent magnetic disk unit, a distance between the tracks, namely, a track pitch is further narrowed in association with the improvement of a recording density of the disk medium. Recording widths of the servo patterns for AGC and off-track detection of the servo surface are also narrowed in association with the narrow track pitch. Even if the offset amount of the head is the same, since the track pitch is narrowed, it is necessary to detect the offset in a wide range. In the conventional servo patterns for off-track detection, however, although a range of the offset positions P2 to P6 in FIG. 2A can be detected, the offset positions P1 and P7 exceeding such a range cannot be detected. That is, as shown in FIGS. 2B and 2H, when both of the pattern read signals V10 and V11 are not obtained, a problem exists as to which of the inner side and the outer side the data head is offset isn't known.
A 2-phase servo system is used to solve the above problem. As shown in FIG. 3A, according to the 2-phase servo system, servo patterns of three tracks having track centers 250-1 to 250-3 are recorded at specific cylinder positions of the data surface for the purpose of offset detection. In this case, an offset amount for the track center 250-2 of the track locating at the center is measured. As shown in FIGS. 3C and 3D, a phase difference corresponding to a width amount of the data head 240 exists between the read signal V10 of servo patterns 220-1 to 220-3 which are offset on the inner side for each track center and the read signal V11 of servo patterns 230-1 to 230-3 which are offset on the outer side. In this case, difference signals (V10-V11) and (V11-V10) of two phases of FIG. 3E can be obtained as difference signals between the read signals V10 and V11. By handling portions shown by solid lines of the difference signals of two phases as effective signals, the offset in a wide range can be accurately detected. In case of the 2-phase servo system, however, as shown in FIG. 3A, an area of three cylinders is necessary when the servo patterns for offset detection are recorded to the data surface. There a problem exists such that the data area is limited and a recording capacity of the disk decreases.